Extraction of metals

ABSTRACT

A method of producing a metal or an alloy from metalliferous material by removing O,S, or N from a solid body of metalliferous material by electrolysis in an electrolytic cell is disclosed. The cell includes a molten halide salt or mixture of halide salts as an electrolyte. The cation of the salt is selected from the group that includes Ca, Ba, Li, Na, K, Mg, Sr, Cs and Y. In one aspect of the invention the method includes conducting the electrolysis under conditions wherein the solid body of metalliferous material is made part of a cathode of the electrolytic cell, the cathode includes a conductor for electrically connecting the cathode with an electrical potential, the conductor has high resistance to chemical attack by the electrolyte at high temperatures, and the conductor is at least partly immersed in the electrolyte. In another aspect of the invention the method includes conducting the electrolysis under conditions wherein the potential applied between an anode and the cathode of the electrolytic cell is chosen such that permanent decomposition of the electrolyte is avoided to an extent that substantial deposition of the electrolyte cation at the cathode is avoided and anode material transport towards and into the cathode is substantially prevented. A cathode for use in the method is also disclosed, which cathode includes a body of metalliferous material distributed around one or more electrical conductors that are substantially inert in the electrolyte at high temperatures and which provide a plurality of reduction zones at the cathode.

FIELD OF THE INVENTION

[0001] The present invention relates to methods of producing metals frommetalliferous materials such as metal oxides.

BACKGROUND OF THE INVENTION

[0002] It is well known to produce metals from metalliferous ores by amethods that include the steps of: 1) concentrating an ore; 2) reducingthe ore concentrate under high temperature conditions in the presence ofa suitable reductant and producing a crude metal; and 3) refining thecrude metal.

[0003] The present invention is concerned with alternative methods ofproducing metals from metalliferous materials that are based on the useof electrochemical cells.

PRIOR ART

[0004] A. A paper entitled “Electrochemical deoxidation of titanium”published in Metallurgical Transactions B, Volume 24B, June 1993, pages449-445 (Authors: T H Okabe, M Nakamura, T Oishi and K Ono).

[0005] The Okabe et al paper discloses an electrochemical method ofremoving oxygen dissolved in titanium.

[0006] The paper reports experimental work on an electrolytic cell thatincluded a cathode of titanium having up to 1400 ppm dissolved oxygenand an anode of graphite. The cathode and the anode were immersed in amolten CaCl₂ electrolyte bath. Electrical potentials between 0 and 6Vwere applied between the anode and the cathode. CaCl₂ was employed toproduce calcium and to facilitate the calcium reaction by decreasing theactivity of the electrolytic by-product CaO. The calcium potential inCaCl₂ was increased at the titanium cathode surface as a result of theapplication of the electrical potential across the anode and thecathode. This resulted in deoxidisation of the cathode by theelectrolytically produced calcium or by calcium of high activity in theCaCl₂. The resulting oxygen ions, which were mainly present in thedeoxidisation product in the electrolyte, reacted at the graphite anodeto form CO or CO₂ gas that was removed from the system.

[0007] B. A paper entitled “Electrochemical deoxidation ofyttrium-oxygen solid solutions” published in Journal of Alloys andCompounds, Volume 237, 15 Apr. 1996, pages 150-154 (Authors: T H Okabe,T N Deura, T Oishi, K Ono and D R Sadoway).

[0008] The Okabe et al paper discloses an electrochemical method ofremoving oxygen dissolved in yttrium.

[0009] The paper describes experimental work on solid yttrium containingdissolved oxygen. The yttrium was placed in a titanium basket cathodeand thereafter immersed in a bath of molten CaCl₂ electrolyte. The CaCl₂electrolyte bath was contained in a titanium crucible and a constantvoltage of between 3.2 to 3.8V was applied between the cathode and agraphite anode submerged in the electrolyte. Electrolysis was carriedout at 1223K (950° C.) for a specified time.

[0010] C. International application PCT/GB99/01781 (patent publicationWO99/64638)(Fray et al).

[0011] The Fray et al International application discloses two potentialapplications of a “discovery” in the field of metallurgicalelectrochemistry.

[0012] One application is the direct production of metal from a metaloxide.

[0013] The other application is the removal of impurities that are“dissolved” in a solid metal. The same basic process is said to beapplicable to both applications.

[0014] The “discovery” in the realisation that an electrochemical methodcan be used to ionise oxygen contained in a solid metal so that theoxygen dissolves in an electrolyte”, compare page 5, lines 14-16. TheInternational application discloses that when a suitably negativepotential in applied in an electrochemical cell with anoxygen-containing metal as a cathode, a reaction occurs whereby oxygenis ionised and is subsequently able to dissolve in the electrolyte ofthe cell.

[0015] The International application discloses an electrolytic cell thatincludes a body of a metalliferous material (such as a metal oxide inwhich impurities are dissolved) as a cathode of the cell. The cathode isimmersed in a molten bath of a suitable electrolyte. A predeterminedelectrical potential that is lower than the decomposition potential ofthe electrolyte is applied between the cathode and a suitable anode(either a separate graphite anode or the electrolyte crucible). Thepotential is chosen such that it has a value that allows a selectedimpurity (i.e. O, S, C or N) to be ionised and thus diffuse through thebody of metalliferous material into the electrolyte where it dissolves.

[0016] The International application lists a substantial number ofmetals that are said to be susceptible for use in the above-describedmethod. These metals are titanium (Ti), silicon (Si), germanium (Ge),zirconium (Zr), hafnium (Hf), samarium (Sm), uranium (U), aluminium(Al), magnesium (Mg), neodymium (Nd), molybdenum (Mo), chromium (Cr),niobium (Nb) or any alloys thereof.

[0017] All of the examples in the International application relate tothe “purification” and/or reduction of titanium, titania, and specifictitanium/aluminium alloys, namely Ti6Al4V, compare pages 9-14 of theInternational application. Example 12 relates to the creation of a Ti—Alalloy starting from a mixture of TiO₂ and Al₂0₃. The ranges of appliedvoltage in the different samples varied from as low as 1.75V (seeexample 2) to 3.3V (compare example 3). Most experiments were conductedat a controlled voltage of 3.0V. Process times varied. Crucibles usedwere made from alumina, graphite, or titanium whereby the anode waseither the crucible or a separate graphite rod. The only electrolyteused in all of the examples was CaCl₂.

SUMMARY OF INVENTION

[0018] Experimental work was carried out at the Minerals TechnologyCentre, Newcastle Laboratories, of the applicant to reproduce theexperiments carried out the above-referenced prior art documents.

[0019] The experimental work resulted in the following findings andinventions.

[0020] 1. Titanium of very low oxygen concentration could be produceddirectly from titania by electrolysis in molten CaCl₂.

[0021] However, cell modification was required to reduce titania in anelectrolytic cell, as the Fray et al International application inparticular is silent on how to set up an electrolytic cell in order toachieve reduction of a good electrical insulator such as titania.Reduction of titania could not be achieved within required parameters byfollowing the experimental set-up disclosed in the Fray et alInternational application.

[0022] Accordingly, a first aspect of the invention in based on therealisation that the type of cathode leads in electrical contact withTiO₂ and CaCl₂ electrolyte severely influence the titania reductionprocess. While there in only a preliminary understanding of themechanism, it is likely that proper selection of the material and thetype of electrical contact will be an important part of the electrolyticcell design specific to metal oxide to be reduced and the electrolyteemployed therefor.

[0023] Accordingly, the first aspect of the invention is a method ofproducing a metal or an alloy from metalliferous material by removing animpurity (I) selected from the group including O, S, or N from a solidbody of metalliferous material by electrolysis in an electrolytic cellthat includes molten halide salt or mixture of halide salts as anelectrolyte, wherein the cation of said salt is selected from the groupthat includes Ca, Ba, Li, Na, K, Mg, Sr, Cs and Y, which method includesconducting the electrolysis under conditions wherein:

[0024] (a) the potential applied between an anode and a cathode of theelectrolytic cell is chosen such that permanent decomposition of theelectrolyte is avoided to an extent that substantial deposition of theelectrolyte cation at the cathode is avoided; and

[0025] (b) the body is made part of the cathode of the electrolyticcell, the cathode includes a conductor for electrically connecting thecathode with an electrical potential, the conductor has high resistanceto chemical attack by the electrolyte at high temperatures, and theconductor is at least partly immersed in the electrolyte; and

[0026] (c) O, S, or N is removed from the cathode and passes intosolution and/or chemically reacts with the electrolyte cation.

[0027] The metalliferous material may contain an oxide, sulfide, carbideor nitride of said metal.

[0028] Preferably the metalliferous material is a titanium-containingmaterial.

[0029] Preferably the impurity in oxygen.

[0030] Preferably the titanium-containing material is titania.

[0031] Preferably the anode is formed from graphite.

[0032] Preferably the electrolyte is CaCl₂.

[0033] 2. Carbon was detected in reduced metal pellets produced in theexperiments.

[0034] While the source of the carbon was the carbon anode employed inthe experiments, the mechanism by which carbon found its way Into thereduced metal is not fully understood. The absolute levels of carbon insome spots of the metal pellet were too high to ignore.

[0035] Accordingly, in a second aspect of the invention there isprovided a method of producing a metal or an alloy from metalliferousmaterial by removing an impurity (I) selected from the group includingO, S, or N from a solid body of metalliferous material by electrolysisin an electrolytic cell that includes molten halide salt or mixture ofhalide salts as an electrolyte, wherein the cation of said salt isselected from the group that includes Ca, Ba, Li, Ha, A, Mg, Sr, Cs andY, which method includes conducting the electrolysis under conditionswherein:

[0036] (a) the potential applied between an anode and a cathode of theelectrolytic cell to chosen such that permanent decomposition of theelectrolyte is avoided to an extent that substantial deposition of theelectrolyte cation at the cathode is avoided and anode materialtransport towards and into the cathode is substantially prevented;

[0037] (b) the body is made part of the cathode of the electrolyticcell; and

[0038] (c) O, S, or N is removed from the cathode and passes intosolution and/or chemically reacts with the electrolyte cation.

[0039] Preferably the cathode includes a conductor having highresistance to chemical attack by the electrolyte at high temperaturesfor connecting the cathode with an electrical potential and theconductor is at least partly immersed in the electrolyte.

[0040] The metalliferous material may contain an oxide, sulfide, carbideor nitride of said metal.

[0041] Preferably the metalliferous material is a titanium-containingmaterial.

[0042] Preferably the impurity in oxygen.

[0043] Preferably the titanium-containing material is titania.

[0044] Preferably the anode is formed from graphite.

[0045] Preferably the electrolyte is CaCl₂.

[0046] 3. In using the above-described inventive methods, it wasconfirmed that Al₂O₃ in contact with a TiO₂ pellet body can be reducedand forms alloys with the reduced titanium.

[0047] 4. It was found that silicon could be reduced from SiO₂ byelectrolysis in molten CaCl₂ when employing the above-described methods.However, chlorine evolution in the cane of SiO₂ reduction was observedto a higher degree compared to TiO₂ reduction.

[0048] 5. Reduction of Al from Al₂O₃ pellets by the above-describedmethod was also attempted.

[0049] It was observed that reduction to Al took place only around thesite of contact between the pellet and the electric leads connecting thecathode to the potential source. The portion of the pellet away from thecathode leads was not reduced at all.

[0050] This observation again suggests that the electrical conductivityof the cathode was a factor that affected the reduction process.

[0051] Accordingly, in another aspect of the invention, there isprovided a cathode for use in the above described methods, wherein thecathode includes the body of metalliferous material distributed aroundone or more electrical conductors that are substantially inert in theelectrolyte at high temperatures and which provide a plurality ofreduction zones at the cathode.

[0052] 6. The mechanisms of removal of oxygen from titanium, titania,yttrium and aluminium-titanium alloys suggested by the Fray et alInternational application and the Okabe et al papers using theelectrolytic methods described are far from clear at present. The Frayet al International application suggests that the mechanism disclosed inthe Okabe papers is incorrect. It is believed that both mechanisms arespeculative insofar as other metals and oxides are concerned. Also,while there is evidence that the type of electrolyte influences theprocess parameters, its properties and role in the presented mechanismsis vague and only qualitative.

Experimental data for the Inventions A. Reduction of Titania FirstExperiment

[0053] The purpose of the first experiment was to confirm (or otherwise)the feasibility of producing metallic titanium from titania by directelectrochemical reduction in molten CaCl₂.

[0054] Specifically, the purpose of the first experiment was to confirm(or otherwise) the set-up described in the Fray et al Internationalapplication. Accordingly, the conditions of the experiment were kept asclose as possible to the conditions in the examples of the Internationalapplication.

[0055] The underlying principle of the process, according to the Fray etal International application, is based on ionization of oxygen in anoxide as a result of applying suitable negative potential to it inelectrochemical cell and subsequent dissolution in the electrolyte.

II. Experimental Method and Equipment

[0056] The experimental set-up is shown in FIG. 1.

[0057] With reference to FIG. 1, the electrochemical cell included agraphite crucible equipped with a graphite lid. The crucible was used asthe cell anode. A stainless steel rod was used to secure electricalcontact between a d/c power supply and the crucible. The cell cathodeconsisted of Kanthal or platinum wire connected at one end to the powersupply and TiO₂ pellets suspended from the other end of the wire. Analumina tube was used as an insulator around the cathode.

[0058] A type B thermocouple, contained in an alumina sheath, wasimmersed in the electrolyte in close proximity to the pellets.

[0059] Two types of pellets were used. One type was slip-cast and theother type was pressed. Both types of pellets were made from analyticalgrade TiO₂ powder. Both types of pellets were sintered in air at 850° C.One pressed and one slip-cast pellet were used in the experiment.

[0060] The experiment was conducted at 950° C. Voltages up to 3V wereapplied between the crucible wall and the Kanthal or platinum wire.

[0061] The power-supply maintained a constant voltage throughout theexperiment. The voltage and resulting cell current were logged usingLabVIEW™ data acquisition software.

III. Experimental Results

[0062] With reference to FIGS. 2 and 3, the constant voltage (3V) usedin the experiment produced an initial current of approximately 1.2 A. Acontinuous drop in the current was observed during the initial 2 hours.After that a gradual increase in the current up to 1 A was observed.

[0063] At the end of the experiment the cell was removed from thefurnace and quenched in water. The solid CaCl₂ was dissolved by waterand the two pellets were recovered.

[0064] SEM images of the cross-sections of the two pellets are shown inFIGS. 4 and 5.

[0065] The presence of virtually pure metallic titanium in both pelletswas confirmed by EPMA analysis. The analysis also showed areas ofpartially reduced titania. The EPMA results are shown in FIGS. 6 and 7.

[0066] Carbon was detected at various locations within the pellets andits content varied up to 18 wt %.

B. Reduction of Silicon I. Experimental Method and Equipment

[0067] The experimental set-up was essentially the same an in the caseof titanium. The cathode consisted of platinum-rhodium wire and SiO₂pellets suspended from the end of the wire.

[0068] The experiment was conducted at 950° C.

II. Experimental Results

[0069] The voltage used in the experiment was 3V, which produced initialcurrent of approximately 1.5 A as shown in FIG. 8. After that a gradualdecrease in the current to 0.65 A was observed.

[0070] The working potential was selected as 3V in order to overcomeresistance and overvoltage. However, chlorine liberation was observed at3V although it is below the theoretical decomposition potential ofCaCl₂, which is 3.25V at 950° C.

[0071] The experiment was terminated after 4 hours. The partiallyreduced pellets were isolated by dissolution of the CaCl₂ in water. Thesurface and interior of the sample were analysed by SEM.

[0072] Analysis of the surface of the pellets showed the presence ofsome oxygen, indicating that there was partial reduction only in theseregions.

[0073] However, the oxygen concentration in these regions was much lowerthan the oxygen concentration in SiO₂—as is shown in FIG. 9.

[0074] The structure of the partially reduced regions of the pellets inshown in FIG. 10. Regions of different phases, such as SiO₂ and2CaO.SiO₂, were detected—see FIGS. 11 to 13.

[0075] Pure unreduced SiO₂ was present in the centre of the pellets.

[0076] Pure Si was identified in the proximity of the platinum leads—asshown in FIGS. 14 to 17.

[0077] Many modifications may be made to the inventions as describedabove without departing from the spirit and scope of the inventions.

1. A method of producing a metal or an alloy from metalliferous material by removing an impurity (I) selected from the group including O, S, or N from a solid body of metalliferous material by electrolysis in an electrolytic cell that includes molten halide salt or mixture of halide salts as an electrolyte, wherein the cation of said salt is selected from the group that includes Ca, Ba, Li, Na, K, Mg, Sr, Cs and Y, which method includes conducting the electrolysis under conditions wherein: (a) the potential applied between an anode and a cathode of the electrolytic cell is chosen such that permanent decomposition of the electrolyte is avoided to an extent that substantial deposition of the electrolyte cation at the cathode is avoided; and (b) the body is made part of the cathode of the electrolytic cell, the cathode includes a conductor for electrically connecting the cathode with an electrical potential, the conductor has high resistance to chemical attack by the electrolyte at high temperatures, and the conductor is at least partly immersed in the electrolyte; and (c) O, S, or N is removed from the cathode and passes into solution and/or chemically reacts with the electrolyte cation.
 2. The method defined in claim 1 wherein the metalliferous material contains an oxide, sulfide, carbide or nitride of said metal.
 3. The method defined in claim 1 or claim 2 wherein the metalliferous material is a titanium-containing material.
 4. The method defined in claim 3 wherein the titanium-containing material is titania.
 5. The method defined in any one of the preceding claims wherein the impurity is oxygen.
 6. The method defined in any one of the preceding claims wherein the anode is formed from graphite.
 7. A method of producing a metal or an alloy from metalliferous material by removing an impurity (I) selected from the group including O, S, or N from a solid body of metalliferous material by electrolysis in an electrolytic cell that includes molten halide salt or mixture of halide salts an an electrolyte, wherein the cation of said salt is selected from the group that includes Ca, Ba, Li, Na, K, Mg, Sr, Cs and Y, which method includes conducting the electrolysis under conditions wherein: (a) the potential applied between an anode and a cathode of the electrolytic cell is chosen such that permanent decomposition of the electrolyte is avoided to an extent that substantial deposition of the electrolyte cation at the cathode is avoided and anode material transport towards and into the cathode is substantially prevented; (b) the body is made part of the cathode of the electrolytic cells; and (c) O, S, C or N is removed from the cathode and passes into solution and/or chemically reacts with the electrolyte cation.
 8. The method defined in claim 7 wherein the cathode includes a conductor having high resistance to chemical attack by the electrolyte at high temperatures for connecting the cathode with an electrical potential and the conductor is at least partly immersed in the electrolyte.
 9. The method defined in claim 7 or claim 8 wherein the metalliferous material contains an oxide, sulfide, carbide or nitride of said metal.
 10. The method defined in any one of claims 8 to 9 wherein the metalliferous material is a titanium-containing material.
 11. The method defined in claim 10 wherein the titanium-containing material is titania.
 12. The method defined in any one of claims 7 to 11 wherein the impurity is oxygen.
 13. The method defined in any one of claims 7 to 12 wherein the anode is formed from graphite.
 14. A cathode for use in the method defined in any one of the preceding claims includes the body of metalliferous material distributed around one or more electrical conductors that are substantially inert in the electrolyte at high temperatures and which provide a plurality of reduction zones at the cathode. 